Christmas week itself is probably the busiest time in the disney calender, and if i had the choice i would avoid it. Having worked at WDW I can assure you the first two weeks of December are lovely and quiet, alot cheaper and yet you still get to enjoy the holidays with the exception of the holiday parade and fireworks at MK which tend to be kept for MVMCP and Christmas week.
If you plan to go during Christmas week, prepare to wait in long lines and if you plan to go to the MK get there early to get in before the crowds really swell. Also if you are going to be there for new years eve please note the traffic afterwards is horrendous. (I lived in lake Buena Vista and it took me 2 hours to get home from work in the early hours of new years day this year from MGM, it usually only took 15min.) Also if you plan to spend xmas day or new years eve at the MK get there when the park opens and stay there. If you leave you will almost definetly will not get back in again, as MK reachs capacity very quickly usually between 10am and 11am. It may be worth thinking about spending NYE at Epcot or DHS and catching the NYE fireworks at MK the night before as they usually do at least 1 practice run during xmas week. Also note DHS only has a big fireworks display a couple of times a year - NYE and 4th July.